He could very well have Giardia from the dirty water he ingested from his little swim. The good news is that many dogs with Giardia have no diarrhea or noticeable symptoms, and if he does develop symptoms or test positive for the parasite, medication should clear it up in a few weeks.
Here’s a good article about it:
https://vcahospitals.com/know-your-pet/giardia-in-dogs
One thing you should note is that it’s possible for humans to get infected with the parasite, so you should use good sanitary measures until you’re sure.